321 
THE ERUPTIONS OF MAY, 
1902, AT THE SOU ERIE RE IN ST. VINCENT. 
anorthite) forms the outer borders of some crystals. Glass cavities are frequent 
(Plate 27, fig. 3). In all their features these felspars resemble the phenocrysts of the 
andesites. 
Augite and hypersthene are about equally common, the former sometimes euhedral, 
hut often anhedral; the latter always occurs in long prisms similar in shape to those 
of the lavas. The augite has a green colour and simple or repeated twinning on 100. 
The hypersthene is pleochroic in the usual tints; one section gave an optic axial 
angle 2E = 132 degrees (which corresponds to 2V = G5 degrees). Parallel growths 
between the pyroxenes occur as in the andesites; they also contain glass enclosures. 
A few scales of dark brown biotite are sometimes present. Olivine occurs in several 
of these norites, but is scarce and is always surrounded by corrosion borders of 
hypersthene (Plate 26, fig. l). The other ingredients are magnetite and apatite. 
In all these rocks there are traces of a matrix between the larger crystals, though 
this is not abundant. In some it is felspathic and consists of small imperfect crystals 
of andesine. In others it is a brown glass, very scanty in amount. This glass 
resembles that of the enclosures and does not seem to be a later injection, as in the 
anorthite-olivine nodules. Most of these rocks, however, contain a small amount of 
micropegmatite (Plate 26, fig. 7), which serves as a groundmass and forms aureoles 
around the felspars. The latter then have borders of oligoclase ; the felspar of the 
micropegmatite is untwinned alkali felspar with lower refractive indices than those of 
the quartz. 
These rocks approach closely in composition and in the peculiarities of their 
minerals to the effusive andesites and differ greatly from the anorthite-olivine 
nodules. The scarcity of olivine, the zonal structure of the felspars, and the presence 
of glassy base in some of them proves that they crystallised at intermediate depths 
and pressures. They represent the rocks which would have been produced had the 
andesitic magma solidified without forcing its way to the surface. 
Sedimentary Rocks. 
Calc-silicate Hornfelses .—The sedimentary rocks ejected by the eruptions of May, 
1902, all occurred in small fragments among the ash and were much contact-altered. 
The commonest were fine grained, pale green or greyish-green calc-silicate rocks. In 
all jn’obability they represent sedimentary beds which lie beneath the volcano. The 
hand specimens are often banded, apparently owing to original bedding; often they 
are crossed by irregular veins, and they may show spots due to local aggregation of 
wollastonite, augite, or other minerals. Similar blocks had been thrown out by 
previous eruptions, and as they are fine grained and tough were used by the Caribs 
in the manufacture of stone implements. 
These rocks contain little carbonates and most have originally been impure siliceous 
and argillaceous limestones. Some consist mainly of wollastonite, with granular 
vol. ccvnr. —a. 2 T 
